The present invention relates to a signal processing circuit such as playback equalizer used in reproducing signals recorded on a magnetic tape.
Referring to FIG. 1, the overall recording/playback frequency characteristic l.sub.12 deviates from the ideal characteristic l.sub.11 due to losses during recording (such as eddy-current loss in the recording head, losses from demagnetization, tape thickness loss, etc.) and losses during playback (loss due to the clearance between the tape and the head, eddy-current loss in the reproducing head, gap loss in the reproducing head, etc.). The overall frequency characteristic l.sub.12 has a peak response at a frequency of about 5 KHz.
In order to flatten the frequency characteristic of a circuit having peaky response, a considerable degree of frequency compensation is necessary in the recording and playback circuits. The operation of compensating frequencies is called equalization, and a circuit for this operation is called an equalizer.
Taking the signal-to-noise ratio and distortion into consideration, the high-frequency region is compensated during recording, and the low-frequency region is compensated during playback. The amount of compensation is varied depending upon the speed of the tape, the kind of tape, the construction and performance of the head, etc. With an object of obtaining compatibility among a variety of magnetic recording/reproducing apparatuses, normal playback compensation characteristics have been determined for each tape seed. FIG. 2 shows examples of recording and playback compensation characteristics for tape speeds of 19 cm/sec and 9.5 cm/sec.
On one hand, the normal playback compensation characteristics are determined by two time constants T.sub.1 and T.sub.2 of a circuit which is made up of a combination of a group of capacitors and a group of resistors. Table 1 illustrates principal standard normal playback compensation characteristics.
TABLE 1 ______________________________________ Normal playback compensation characteristics Tape speed 38 19 9.5 4.8 Time constant T.sub.1 T.sub.2 T.sub.1 T.sub.2 T.sub.1 T.sub.2 T.sub.1 T.sub.2 ______________________________________ BTS 3180 35 3180 50 -- -- -- -- CCIR -- 35 -- 70 -- -- -- -- DIN -- 35 -- 70 -- -- -- -- -- -- 3180 50 3180 90 3180 120 IEC -- 35 -- 70 3180 90 1590 120 JIS -- -- 3180 50 3180 90 -- -- NAB 3180 50 3180 50 3180 90 3180 90 RIAA -- -- 3180 50 3180 90 1590 120 ______________________________________ Tape speed [cm/sec], time constant [microseconds]
On the other hand, in the field of magnetic tape recording/playback apparatuses, not only the open reel-type and cassette-type apparatus having different tape speeds have been developed, but also normal tapes, chrome tapes and metal tapes having different characteristics have been developed.
Even in the cassette type magnetic tape recording/playback apparatus in which the standard tape speed is 4.8 cm/sec, the tape is sometimes run at half speed or double speed. Therefore, playback compensation characteristics are required for the magnetic tape playback equalizer to respond to both various tape speeds and various kinds of magnetic tape.
FIG. 3 shows playback compensation characteristics suited for this sort of magnetic tape playback equalizer, in which the gain of playback compensation characteristics is about 40 dB at a frequency f.sub.1 of 50 Hz which corresponds to a time constant T.sub.1 of 3180 microseconds, and the gain decreases by 20 dB per octave in the frequency bands higher than f.sub.1. When any one of the frequencies f.sub.21, f.sub.22, f.sub.23, f.sub.24, f.sub.25 and f.sub.26 corresponding to the respective time constants T.sub.2 of 35 .mu.S, 50 .mu.S, 70 .mu.S, 120 .mu.S and 200 .mu.S, is selected, the gain assumes flat characteristics in the frequency band higher than the selected frequency. Thus, equalization can be provided for different tape seeds and tape types by appropriate selection of the time constants to control the playback compensation characteristics.
In the conventional magnetic tape playback equalizer, a negative feedback circuit is constructed based upon a negative feedback-type amplifier circuit which is made up of a combination of a group of capacitors and a group of resistors. To change the playback compensation characteristics in response to the tape speed and the kind of tape, the resistances of the resistor group in the negative feedback circuit must be changed. Therefore, it was not possible to form the negative feedback circuit for such a conventional playback equalizer in a monolithic semiconductor integrated circuit.
Prior to filing the present application, the inventors of the present invention considered a system in which the time constant T.sub.2 at a high frequency f.sub.2 in the playback compensation characteristics is specified by means of a group of semiconductor diffused resistors and a group of semiconductor switching elements formed in a monolithic semiconductor integrated circuit and by a capacitor array outside the monolithic semiconductor integrated circuit. As is well known among those skilled in the art, however, the resistance of diffused resistors formed in a monolithic semiconductor integrated circuit have errors of .+-.20% that stem from the process for manufacturing the integrated circuits. This fact causes the time constant T.sub.2 at a high frequency f.sub.2 in the playback compensation characteristics to vary greatly, so a system of this type is not practical for providing accurate playback compensation.